The present invention relates to a composite panel which has two outer layers and between these a core comprising a mixture of a granular filler material and an adhesive.
Composite panels of the above mentioned kind, comprising metallic or non-metallic cover sheets joined over their whole inner surfaces by means of an adhesive to a central, inorganic core, are already known. The core materials used in the known composite panels can be inorganic and incombustible. All such composites known to date suffer, however, from certain fundamental disadvantages.
Inorganic core materials and adhesives are indeed incombustible but, because they lack elasticity, they are not able to withstand the deformation resulting from mechanical stressing or thermal dilation of the outer layers. The outer layers of such composites therefore decohere or delaminate in a brittle manner. Also they are usually sensitive to water vapor, or they absorb moisture, which can then lead to corrosion of the metallic cover sheets, in particular if used in connection with alkaline substances.
If the inorganic core materials contain hard mineral components or asbestos fibers, machining operations such as sawing and drilling can be carried out only with special tools; the machining may also be made more difficult by the formation of dust containing components which could be toxic.
Organic or mainly organic materials such as core compositions which have plastics in their make up can not provide the non-combustibility required, as the effect of fire or high temperatures causes large quantities of combustible gases to be released as a result of the decomposition of organic components.
It is known that the combustibility of organic substances such as artificial resins can be reduced significantly by the inclusion of the maximum possible amounts of aluminum hydroxide Al(OH).sub.3 or hydrated aluminum oxide.
The effect here is due partly to the reduction in the calorific value per kg of mixture, corresponding to the amount of filler material added. Another factor is that approx., 1880 kJ of energy per kg Al(OH).sub.3 is used to dehydrate and heat up Al(OH).sub.3 which contains up to 33 wt% of chemically bonded water i.e. as in the equation ##EQU1## Furthermore, the water vapor acts as a protective gas, excluding oxygen from the region of combustion. The formation of a surface layer of filler material and carbonized products also protects and insulates the lower lying layers from the source of ignition and therefore reduces further the decomposition into combustible products.
In many countries criteria have been worked out and standardized to allow materials containing only small amounts of organic substances to be classified as non-combustible building materials or components. In Germany for example, to meet the requirements for classification as non-combustible, it must be shown (in accordance with the German Specification DIN 4102) that the lower calorific value of the building material is not more than 4200 kJ/kg of material or, in building components, the quantity of heat given off under first test conditions is less than 16,800 kJ/m.sup.2. In Switzerland and France the evidence for incombustibility and the classification for incombustible components are based on similar conditions.
The object of the present invention is to develop a composite panel which can be judged as being in the class of non-combustible building components in terms of the criteria and specifications worked out by the above mentioned countries, and by means of which at the same time the above mentioned disadvantages associated with the known composite panels using inorganic core materials and adhesives are avoided.